Provide examples and guidance and notes at the point of need. Have you ever got half-way through a form and suddenly come to a question that asks you about something you do not understand or have no idea about where to find the answer? Its even more infuriating if the question says something like.

In paper forms the specific sub-questions cannot be hidden from the user when they are not relevant to them - but in interactive forms this is relatively easy to do. Use progressive revealing as much as possible in your form design to shield your users from questions they do not need to see.